1. Research Institute for Magnetoelectronis and Weak Magnetic-Field Detection, College of Science, China Three Gorges University, Yichang 443002, People's Republic of China
2. Department of Physics, Chungbuk National University, Cheongju 28644, South Korea
3. Research Institute for Nanoscale Science and Technology, Chungbuk National University, Cheongju 28644, South Korea
4. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
5. Department of Physics, University of California at Santa Cruz, Santa Cruz, California 95064, USA